A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Shoemaker
Date: 2017 Jul 21, 06:40 -0700
While not regarded as being very accurate, one method of calculating longitude is to take the time difference from the meridian passage of the sun at the Greenwich and the noon sight at the navigator’s location. The equation of time must also be included. I have been experimenting with this using exact locations and using the Geocentric Positions of Major Solar System Objects - http://aa.usno.navy.mil/data/docs/geocentric.php – to determine the equation of time. But I am finding small discrepancies in longitude. An example would be for today, 21 July 2017. The equation of time at 1200 Z is -6m 25.4s. So the sun is on the meridian at 12h 06m 25.4s at Greenwich. 6 hours later, the equation of time is -6m 26.1s and at 90 degrees west the sun is on the meridian at 18h 06m 26.1s. You would think that subtracting that time from the time at Greenwich would be 6h 00m 00s. Multiply that by 15 degrees per hour would give exactly 90 degrees longitude. But in reality the time difference is 6h 00m 00.7s. That multiplied by 15 degrees per hour gives 90 degrees 00 minutes 10.5 seconds longitude. Not much of a difference and that level of precision isn’t possible or necessary in a noon sight. But why the difference? Would it be safe to say that because of the equation of time, the sun isn't really moving at exactly 15 degrees per hour?